Various methods for producing particles carrying radioactive nuclides are known. One method, disclosed in U.S. Pat. No. 3,334,050, comprises the application of high temperatures for sealing nuclides into the interstices of ion exchange cores by carbonizing the core. Using this method it has been found difficult to obtain a high yield of uniform and desired size cores because of the difficulty in controlling shrinkage of the particles. In addition, certain nuclides such as Mercury.sup.203 or Iodine.sup.125 are extremely volatile at temperatures used for carbonization and thus losses of these nuclides would be expected to occur. Furthermore, it has been found in practice that particles produced in this manner when utilized as an injectable preparation in animal research tend to agglomerate both in an injectable preparation and in vivo thus comprising test results.
Another technique that is described in U.S. Pat. No. 3,492,147 uses a non-reactive or inert substrate (e.g., sand, glass, etc.) to which a monomeric coating containing radioactive nuclides is applied and is polymerized by extraction of a catalyst from an acid bath which is contacted with monomer coated particles. It has been found in practice that with this process substantial undesired bulk polymerization occurs, which limits the usefulness of the product.
A further process of the prior art involves the incorporation of .sup.51 Cr acetylacetonate (a chelating agent) into polystyrene and polystyrene vinyl lattices in toluene (non ion exchange resin) by a process called emulsion polymerization. This process tends to produce particles of very small dimensions (about 0.1 to 1.5 microns) which are too small for convenient use in animal circulatory studies.
Many of the problems associated with the prior art were solved by the development of a tracer particle having an ion exchange resin core with a controlled thickness polymer coating as described in U.S. Pat. No. 4,107,283. These tracer particles were found to be non-agglomerating in an injectable suspension, and when used in vivo or when stored in dry form. Although the controlled thickness polymer coating provides substantial non-leaching characteristics to the tracer particles described in U.S. Pat. No. 4,107,283, there still remains undesirable leaching of some radionuclides from the ion exchange resin core, particularly under certain conditions such as storage in solution at room temperature or in vivo use at body temperatures. Thus, these particles when labelled with particular nuclides degrade rapidly in vivo in certain types of animal studies making them unsuitable for use in various experiments.
Thus, there remains a need for improved tracer particles having improved stability against leaching of the radionuclide label during storage and in vivo.